Two notes on using jit.dx.grab

Below I would like to share two issues I encountered on using the
jit.dx.grab. As it is, all information are windows specific.

First, I have found that the object will lead to a crash and closing
Max/Jitter if you try to change the video device in case there is actually
no device available on your PC. This may not seems to be a problem in most
case, but if in case you want to reuse some of your old code, you will find
it never reopenable and you lose it. In jit.qt.grab, it is safe. I guess a
check of enumerated device internal in Jitter will solve the problem.

Second, you can only change your video input (among a device) when you have
already opened jit.dx.grab. It was an annouying problem at the very
beginning when I wanted my patch to autostart and run and I had loadbangs to
wake up everything but the video was always blank. Be noted.